(a) Field of the Invention
The present invention relates to an optical device having a carrier-depleted layer.
(b) Description of the Related Art
An optical integrated device including a distributed-feedback (DFB) laser diode and an electro-absorptive optical modulator (referred to as simply EA modulator hereinafter) is generally referred to as EA-DFB module or EAML (electro-absorptive-modulated laser) module. The EA-DFB modulator has advantages over the conventional optical device having an external optical modulator in that the EA-DFB module has a smaller occupied area and can be fabricated with a lower cost. Thus, the EA-DFB module attracts more attention in the field of wavelength-division-multiplexing communication system. In particular, the EA-DFB module is noticed as a key device for implementing a light source in a metropolitan optical communication system.
The EA modulator includes an absorption layer having a quantum well structure, and uses an electro-absorption effect wherein the absorption coefficient for laser changes depending on the electric field applied therein. The operational principle of the EA modulator is such that the absorption layer is applied with a reverse bias voltage by using a quantum confined stark effect to shift the spectrum end of absorption by excitons toward a longer wavelength side, i.e., lower energy side, thereby absorbing the laser and thus reducing the laser intensity emitted by the DFB laser diode.
In general, the EA-DFB module has a buried structure wherein a Fe-doped InP (Fe-InP) electron trapping layer is buried on both sides of mesa stripes. The Fe-InP electron trapping layer is subjected to carrier depletion by the Fe dopant forming a deep energy level, which achieves a lower capacitance for the EA modulator. Some EA-DFB modules having the above structure are capable of operating for an effective optical modulation at rates as high as several tens of giga bits per second (Gbps).
A conventional EA-DFB module will be described hereinafter with reference to FIG. 7 showing the layer structure of the EA-DFB module, and to FIGS. 8A and 8B taken along lines VIIIa—VIIIa and VIIIb—VIIIb, respectively, in FIG. 7.
The EA-DFB module 40 has a semi-insulating planar-buried-heterostructure (SI-PBH) including a buried Fe-InP layer as an electron trapping layer The EA-DFB module 40, as shown in FIG. 7, includes a DFB laser diode formed in the DFB laser area 40A and an EA modulator formed in the EA modulator area 40B. The DFB laser diode and the EA modulator are integrated in a monolithic structure and co-axially arranged on a common n-InP substrate 41 in the direction of optical axis of the waveguide.
The DFB laser area 40A, as shown in FIG. 8A, has a layer structure including an n-type InP (n-InP) lower cladding layer 42, a multiple-quantum-well separate-confinement-heterostructure (MQW-SCH) active layer structure 43, a p-InP spacer layer 44, a diffraction grating 45, and a p-InP upper cladding layer 46, which are consecutively formed on the n-InP substrate 41. Among the layers in the layer structure of the DFB laser area 40A, the p-InP upper cladding layer 46, diffraction grating 45, p-InP spacer layer 44, MQW-SCH active layer structure 43, n-InP lower cladding layer 42 and top portion of the n-InP substrate 41 are configured as a mesa stripe 50.
The EA modulator area 40B, as shown in FIG. 8B, has a layer structure including an n-InP lower cladding layer 47, a MQW-SCH absorption layer structure 48, and a p-InP upper cladding layer 49, which are consecutively formed on the n-InP substrate 41. Among the layers in the layer structure of the EA modulator area 40B, the p-InP upper cladding layer 49, MQW-SCH absorption layer structure 48, n-InP lower cladding layer 47, and top portion of the n-InP substrate 41 are configured as a mesa stripe 51 extending from the mesa stripe 50 of the DFB laser area 40A.
A current blocking structure is formed on both sides of the mesa stripes 50 and 51 in contact therewith, the current blocking structure including a semi-insulating Fe-InP electron trapping layer 52 and an n-InP hole blocking layer 53, which are consecutively buried on both the sides of the n-InP substrate 41.
A p-InP upper cladding layer 54 and a p-InGaAsP contact layer 55 are consecutively formed on top of the mesa stripes 50 and 51 and the n-InP hole blocking layer 53. A p-side electrode 56 is formed on the p-InGaAsP contact layer 55 in the DFB laser area 40A, whereas another p-side electrode 57 is formed the p-InGaAsP contact layer 55 in the EA modulator area 40B. A common n-side electrode 58 is formed on the bottom surface of the n-InP substrate 41 in the DFB laser area 40A and EA modulator area 40B.
As shown in FIG. 7, a deep trench 59 is formed outside the mesa stripes 50 and 51 in a spaced relationship therewith by etching the p-side electrodes 56 and 57, p-InGaAsP contact layer 55, p-InP upper cladding layer 54, n-InP hole blocking layer 53, Fe-InP electron trapping layer 52, and top portion of the n-InP substrate 41. The deep trench 59 reduces the parasitic capacitance of the EA-DFB module 40.
Among the layers in the layer structure in the vicinity of the boundary between the DFB laser area 40A and the EA modulator area 40B, the p-side electrodes 56 and 57, p-InGaAsP contact layer 55 and top portion of the p-InP upper cladding layer 54 are etched to configure a shallow trench 60 acting as an isolation trench. The isolation trench 60 electrically isolates the p-side electrode 56 from the p-side electrode 57.
The p-InP upper cladding layer 54 of the EA-DFB module has a relatively higher carrier density of about 1×1018 cm−3, which affords a lower device resistance for the DFB laser area 40A. In addition, the n-InP hole blocking layer 53 has a relatively higher carrier density of 1×1018 cm−3 to 1×1019 cm−3, which affords a lower threshold current and a higher current-to-light conversion efficiency for the DFB laser area 40A.
However, the relatively higher carrier densities of the n-InP hole blocking layer 53 and the p-InP upper cladding layer 54 suppress expansion of the carrier-depleted layer generated in the vicinity of the boundary between the n-InP hole blocking layer 53 and the p-InP upper cladding layer 54 in the EA modulator area 40B. This results in a higher electric field across the boundary, degrading the reverse voltage tolerance in the EA modulator area 40B. Thus, the p-n junctions of some EA modulators cannot withstand the reverse bias voltage needed for performing an effective absorption operation, i.e., needed for obtaining a desired extinction ratio. The term “extinction ratio” as used herein means a ratio of the minimum laser intensity to the maximum laser intensity obtained at the output of the EA modulator.
In addition, the higher electric field may damage the p-n junction of the current blocking structure of the EA modulator area 40B; even if the EA-DFB module has a reverse voltage tolerance sufficient for obtaining a desired extinction ratio.